Cloud Effect In Blender to Unity
You could create a cube, subdivide and smooth it:

Give it several shapekeys:

In Object mode, select a shapekey, push its Value up to 1 and sculpt:

Sculpt the other shapekeys, animate them:

Or you could create your mesh with metaballs that you convert to mesh:

Then give it a Displace modifier with a Cloud texture and an empty as Object:

In the Displace modifier, save several versions As Shapekeys:

Now you can animate the shapekeys, also sometimes do some Shrink/Fatten to inflate the cloud a bit: